Exemplary embodiments of the present invention relate to spark plugs for use in internal combustion engines, and, more particularly, to spark plugs having an electrode which includes a tip portion that is capable of being resistance welded to nickel-based electrodes and resistant to wear in oxidizing conditions at high temperatures.
Spark plugs are widely used to ignite fuel in internal combustion engines. The electrodes of a spark plug are subject to intense heat and to an extremely corrosive environment generated by the formation of sparks and combustion of the air/fuel mixture. An electrode suffers whenever there is discharge during sparking, which can result in very high local temperatures and wear because of sputtering, melting, ablation, splashing, and particle erosion. In addition, the primary cause of wear for spark plugs is the failure of the electrodes caused by exposure to oxidizing conditions at elevated temperatures with high-sparking voltages. Oxidation proceeds from the surface of a healthy electrode, and spark discharge can remove weakened oxidized scales from the electrode surface causing wear and significantly reducing the life of the spark plug.
Thus, as an electrode becomes hotter, the speed of both sparking wear and oxidation wear accelerates. With combustion engines moving toward more lean burn to reduce emissions, and with cylinder pressures, compression ratios, and ignition voltages increasing, the reliability and lifetime performance of spark plugs are critical to accommodate further advances in engine development. Therefore, to provide durability and erosion resistance, the material used for spark plug electrodes should have a high melting point and be highly resistant to oxidation to withstand the high temperatures and corrosive environment that result from the chemical reactions between air, fuel, and fuel additives within the combustion chamber.
The manufacture of copper (Cu) and nickel (Ni) electrodes for spark plugs has been accomplished in various ways. For instance, U.S. Pat. No. 4,705,486 (“the '486 patent”), entitled “Method for Manufacturing a Center Electrode for a Spark Plug” the contents of which are incorporated herein in their entirety by reference thereto, discusses a method for manufacturing a center electrode that provides a degree of longevity for the spark plug. The center electrode is made from a good heat conducting material such as copper surrounded by a jacket of a corrosion resistant material such as nickel. Nickel, however, is susceptible to selective oxidation at high temperatures, which limits the life of the spark plug. Moreover, after a period of operating at higher temperatures in recirculation gases, corrosion/erosion can occur in the nickel-based electrode. Once corrosion has taken place, the electrical flow path will deteriorate and result in lower fuel efficiency.
To resist erosion caused during service in oxidizing conditions at elevated temperatures with high sparking voltages, heat- and wear-resistant tips consisting of high-cost precious metal alloys can be added to the discharge end of the spark plug electrode. While these spark plug electrode tips are tougher and more erosion resistant than the balance of the electrodes, they are situated at the points at which the spark crosses over between the sparking electrodes and therefore are among the most critical working parts of a spark plug.
The '486 patent, for example, provides a method of manufacturing an electrode for a spark plug in which a platinum (Pt) tip is attached to a body composed of a nickel alloy such as Inconel that is disposed about a copper core. Other illustrative examples of publications relating to various wear-resistant spark plug electrode tips, and to spark plugs including such electrode tips, may be found in U.S. Pat. Nos. 6,597,089, 6,166,479, 6,094,000, 6,071,163, 5,998,913, 5,980,345, 5,793,793, 5,973,443, and 5,456,624; PCT Pub. No. WO 01/18925; and U.S. Pat. Pub. Nos. 2004/0027042 and 2002/0171346, the contents of each of which are incorporated herein in their entirety by reference thereto
Some wear-resistant spark plug electrode tips incorporate precious metals such as platinum because they provide reasonably good resistance to oxidation and erosion under exposure to a combustion chamber environment. Platinum, however, is susceptible to intergranular cracking and attack by oxidation and lead found in certain fuels being used with internal combustion engines. Progressive oxidation and crack growth can result in a substantial increase in electric resistance and thus breakdown voltage for ignition to continue. The erosion and deterioration of the platinum tip portion causes the sparking gap to widen, thus weakening the spark that the spark plug produces. Furthermore, platinum is a very expensive raw material, as are the other noble metals, and it is therefore advantageous to strictly control the amount of noble metal which is incorporated into each spark plug.
Iridium (Ir) has shown excellent resistance to attack by a wide range of molten metals. For instance, iridium is superior to platinum in withstanding attack by lead. Furthermore, iridium can provide superior wear resistance with a narrower center diameter, which allows for improved ignition. The coefficient of thermal expansion of iridium, however, differs significantly from nickel. Under high thermal stress, this difference can cause weakening or fracture to occur at the area where an iridium-based electrode tip portion and a nickel-based electrode are joined as the tip portion and electrode heat up during use of the spark plug, and may even lead to physical separation of the noble metal and base metal. Thus, iridium and its alloys have heretofore been extremely difficult to resistance weld or otherwise secure to an electrode comprised of nickel-based substrate alloys without experiencing gradually cracking and/or breaking at the joint between these components, particularly in side electrodes where the thermal stresses are most severe.
Although various designs for spark plugs having wear-resistant electrode tips are known, the inventors herein have recognized a need for a spark plug having an electrode construction that allows for a long life of operation before the spark plug requires replacing, is highly wear-resistant and resistant to oxidation at high operating temperatures, and can provide a reliable, oxidation-resistant weld between the tip portion and the substrate portion.